Among the open questions is how the vice president’s entry would possibly affect Clinton’s bid for the nomination. So ABC News dove into the details of recent polling to see what people have been saying about the two candidates. Here’s what we found:

When Clinton goes head to head in the polls with Biden on who’s a stronger leader, she usually comes out on top. This September, though, when Quinnipiac University asked respondents whether each candidate had “strong leadership qualities,” Biden was better off, although only by one point.

Polls from April, May, July and August of this year tell a different story, with Clinton leading. It remains to be seen whether the recent speculation surrounding Biden’s candidacy makes him seem more presidential to voters in the long term, or if this spike is just a fluke.

Biden’s Favorability Numbers Are Higher

Overall, registered voters just like Biden better. In a September Quinnipiac poll, when asked if they had a favorable or unfavorable view of each candidate, only a third of respondents had an unfavorable view of Biden, while half had an unfavorable view of Clinton. Indeed, more people had an unfavorable view of Clinton in September than a favorable one: Her net rating for the category was a negative 14.

But Clinton hasn’t always struggled to be well-liked. In December 2013, poll respondents actually liked her more than Biden. Her favorability among voters has fallen since then but, in general, a majority of Quinnipiac respondents have held a favorable view of her; until, that is, she declared her candidacy in April. It’s possible that the same thing could happen to Biden if he jumps in.

Respondents Believe Biden Is More Honest and Trustworthy

Clinton has also been struggling throughout the race to convince voters she is someone they can trust, and so far she hasn’t found much success. In September, when Quinnipiac asked voters whether each candidate was "honest and trustworthy," there was a 30-point margin between Clinton and Biden’s scores. Only 32 percent of respondents said they believed Clinton was honest and trustworthy, while voters trusted Biden much more: 62 percent said he held those qualities.

And That He Cares More About Their Problems

When Quinnipiac asked voters in September whether Biden “cared about the needs and problems” of people like them, 61 percent of them said “yes.” But only 43 percent of them thought the same thing about Clinton.

This is par for the course for Clinton. In April, only 47 percent of respondents thought she cared about their needs and problems, showing that she has a ways to go before she can really connect with voters.

… But Clinton Is Still Polling Better in the Horse Race

The gap between Biden and Clinton in the national horse race has been narrowing recently. In September, an ABC News-Washington Post poll showed Clinton leading Biden by only 21 points, a dramatic drop from a previous 51 point lead in July.

Despite these difficulties, though, Clinton has led the horse race since polling started last year. In May, an ABC News-Washington Post poll showed Clinton beating Biden by a whopping 54 points.

The 21-point gap between Clinton and Biden could continue to narrow if Biden jumps in. Or it’s possible that, like Clinton in April, he could experience a drop in favorability numbers.

It would possibly take a while to see the effects of his joining the race but, this time, Biden would be the one waiting to find out.